1. Field of the Invention
The present invention relates to analog to digital conversion (ADC) at high bandwidths, such as bandwidths in the range from about 100 MegaHertz (MHz, 1 MHz=106 cycles per second) to about 1000 GigaHertz (GHz, 1 GHz=109 cycles per second) centered at an arbitrary frequency.
2. Description of the Related Art
Electronic analog-to-digital converters (ADCs, also called “digitizers”) that can directly capture signals with bandwidths over 10 GHz require over 20×109 samples per second (SPS) and are currently limited to fewer than 7 effective resolution bits. Only modest improvements in performance are anticipated with current approaches. The current approaches include some electronic and photonic techniques that leverage the ability of currently available 0.1 Giga-samples per second (GSPS, 1 GSPS=109 samples per second) ADCs to achieve roughly 12 effective bits. High bandwidth electronic techniques include sample-interleaved ADCs, but their performance is either limited by clock jitter, component non-uniformity, and calibration issues.
Previous approaches to photonic-assisted ADC include ultra fast pulse sampling, which takes advantage of stable pulse trains from mode-locked lasers, and time-stretched waveform processing, which exploits the ability of dispersive elements to temporally stretch modulated broadband chirped pulses. In time stretching, a microwave waveform is intensity modulated onto a frequency chirped optical pulse, which is then stretched by a highly dispersive element, such as a doped optical fiber, thus reducing the pulse's intensity modulation bandwidth. The resulting signal can be captured using a lower bandwidth electronic ADC. Bandwidth enhancements (time-stretch factors) significantly greater than 10 and time-bandwidth products significantly greater than 1000 are difficult to achieve with currently available devices.
Based on the foregoing, there is a clear need for ADCs that are capable of directly capturing signals with high bandwidth and providing more effective resolution bits than is achieved with current approaches. In general, there is a need for ADCs that do not suffer all the deficiencies of current and prior approaches.